Conventionally, in the case where a variable scaling of an input image is performed by a digital copying machine, namely, the input image is enlarged or reduced by a digital copying machine, it is enough to select either of a nearest neighbor interpolation and interpolations of first through third degrees per page.
In the nearest neighbor interpolation, an image is divided into blocks composed of a plurality of picture elements, and when enlarging the image, the data of the picture element existing in the nearest neighborhood of a target picture element is used as data of the target picture element. The enlarging method by means of the nearest neighbor interpolation has a preferable characteristic that an edge of a character becomes clear.
In addition, in the interpolation of first through third degrees, data of a target picture element in divided blocks are determined by using an computing mean of the data of the nearest neighbor picture element of the target picture element and data of another neighbor picture element, which is obtained by formulas of the first through third orders so that an image is enlarged. This method is also called as a linear interpolation. More specifically, the data of the nearest neighbor picture element of the target picture element and the data of another neighbor picture element are respectively weighted with an inverse number of a distance from the target picture element and a weighted mean is obtained. The data obtained from the weighted means is specified as data of the target picture element.
The enlarging method through the interpolations of first through third degrees is preferable as the enlarging process in the case of processing an image in which density gradually changes like a photographic image because new data of a picture element interpolated between the respective picture elements (interpolation picture element data) become means of picture element data existing on both sides adjacent.
However, in the case where two or more types of image data, such as characters, photographs and mesh dots, coexist on one page of image, if a variable magnification of such an image is processed by either of the nearest neighbor interpolation and the interpolations of first through third degrees, image quality of either of the characters, the photographs, etc. is deteriorated.
Therefore, Japanese Unexamined Patent Publication No. 6-286221/1994 (Tokukaihei 6-286221) discloses a printing method which is capable of changing a variable scaling process according to types of data in the case where characters, photographs, mesh dots, etc. coexist on one page.
In this printing method, input data such as character information such as character codes and form information are inputted from a host as an external device into a buffer memory of a printer. The printer creates a character pattern and a form pattern according to the information of the inputted data so as to print an image.
In the case where enlarging or reducing process is executed prior to printing, when a special enlarging or reducing method is cataloged into the printer according to the character pattern, etc., variable scaling is performed by the enlarging or reducing process. Therefore, the variable scaling process can be performed according to each type of data.
However, since the above conventional image processing method is applied to a printer, and the variable scaling process of such a printer can be previously judged by information from an external host that characters, vector drawings and bit images is inputted as image data. Therefore, in image input and output devices such as a copying machine, the above-mentioned image processing method has a disadvantage that in the case where document image is read by a scanner and characters, photographs and mesh dots coexist on one page of the read document image, a variable scaling process cannot be performed according to the type of image data.